Salmonellae are Gram-negative, facultative anaerobic, motile, non-lactose fermenting rods belonging to the family Enterobacteriaceae. Salmonellae are usually transmitted to humans by the consumption of contaminated foods and cause salmonellosis.
Salmonellae have been isolated from many animal species including, birds, cattle, sheep, pigs, dogs, cats, horses, donkeys, seals and reptiles. Ninety-five percent or more of the Salmonella serovars (ser.) isolated from food producing animals belong to Salmonella enterica subspecies enterica (S. enterica), with Salmonella ser. Typhimurium (S. Typhimurium), Salmonella ser. Choleraesuis (S. Choleraesuis), Salmonella ser. Derby (S. Derby), Salmonella ser. Infantis (S. Infantis), Salmonella ser. Bredeney (S. Bredeney), Salmonella ser. Rissen (S. Rissen), and Salmonella ser. Anatum (S. Anatum), as the most common serovars in pigs.
Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Control of salmonellosis is important to avoid potentially lethal human infections and considerable economic losses for the social security and animal husbandry industry.
For said reasons, Salmonella monitoring programs aiming to reduce pork-related salmonellosis in humans are imposed all over the world. In a substantial proportion of these monitoring programs, the amount of Salmonella-directed antibodies in the blood or meat juice of pigs is used as the sole tool to categorize pig farms as low-risk or high-risk farms regarding their Salmonella status. The status of high-risk farm with respect to Salmonella can have serious implications for the national and international economical position of the company.
There has been a long history of the use of live attenuated Salmonella vaccines as safe and effective vaccines for the prevention of salmonellosis in animals and humans. Indeed, the live attenuated oral typhoid vaccine, Ty21a (Vivotif), manufactured by the Swiss Serum Vaccine Institute, has proved to be a very successful vaccine for the prevention of typhoid fever and has been licensed in many countries including the US and Europe. Pig farmers and veterinarians are reluctant to use vaccination in the control of Salmonella in pigs since vaccination with current vaccines cannot be distinguished from Salmonella infections using the serology based monitoring programs.
It is therefore desirable to develop a vaccine that would induce a good immune response, and that is serologically distinguishable from animals infected with Salmonella. 